Traditionally, a radar signal consisted of a burst of 100 to 1000 cycles of a fixed frequency sine wave. As an example, if the carrier frequency is 333 MHz, then one cycle is approximately 3 nanoseconds long, and the duration of a burst (i.e., pulse) might be between 300 nanoseconds (i.e., 100 cycles) and 3 microseconds. An electromagnetic wave travels approximately 100 meters in 333.3 ns. If the largest linear dimension of a target is small compared to 100 meters, then the target acts like a point scatterer and the returned signal will have essentially the same amplitude versus time variation as the transmitted burst. The slight distortions of the returned signal due to the finite extension of the target are called the radar signature. In principle, the radar signature can provide information about such features as the shape of the target and the material composition of its surface.
When a radar transmits a character consisting of a sequence of pulses, the received character has almost the same time variation as the transmitted character. However, this situation changes as the pulses are made shorter. For instance, a pulse duration of 33.3 ns implies that the wave will travel a distance of 10 meter during the duration of the pulse. Few targets are like point scatterers for such short pulses. As a result, the pulse, or the character consisting of a sequence of pulses, becomes heavily distorted, which is advantageous since it implies that more information is received about the shape and composition of the target. When a carrier frequency is used, the signal is still recognizable from the frequency of the carrier, despite the distortions. This is due to the particular feature of (periodic) sinusoidal waves, whereby the sum of any number of sinusoidal functions all with the same frequency, but different amplitudes and phases, always yields a sinusoidal function of that frequency. Hence, a burst with enough sinusoidal cycles to allow detection can be recognized by their carrier frequency regardless of the distortions.
Recently, radars have been developed theoretically and experimentally that do not use a sinusoidal carrier. These radars typically use pulses with a duration of lns or less. A lns pulse would require a carrier frequency of 100 GHz or greater, if conventional technology were used. But the propagation features of the atmosphere make such high carrier frequencies undesirable and unacceptable. Yet the short duration of the pulses yields enormous information due to the distortion of the pulses returned by the target. It is evident that short bursts are desirable, but employing a fine structure to mark them is ineffective as means that permit the selective reception of the wanted, distorted, signal in the presence of unwanted signals and noise. This invention teaches a method for using short duration pulses in a coarse structure that marks them for selective reception so that fine structure marking is not required. General background on carrier free radar and pulse position coding is available in the book by H. F. Harmuth, Nonsinusoidal Waves for Radar and Radio Communication, Academic Press, New York 1981, which is hereby incorporated by reference.